Question

I know what ArrayList<T> is used for, but when should I use ArrayList<?> ? can you explain with example? thanks.

Answer 1

As far as I’ve been able to tell, ArrayList<?> basically tells the compiler:

Yes, I know that there is a generic version of ArrayList available to me, but I really, genuinely don’t know what kind of objects I’m expecting to be in this one. So don’t give me warnings that I’m not using generics the way I should be.

Update

I just learned that there is a real difference between using Raw Types (ArrayList) and generics with wildcards (ArrayList<?>) that goes beyond avoiding compiler warnings. Apparently once you declare something as a Raw type, no generics will work on any methods of that type, even if the generics weren’t based on the type you omitted. See here for an example.

So while my original answer was generally correct, I thought it would be important to mention that using ArrayList<?> instead of ArrayList is more than just a matter of removing compiler warnings.